Alteration of O6-methylguanine-DNA methyltransferase in colorectal neoplasms in sporadic and familial adenomatous polyposis patients

DNA repair failure is known to be a critical event during carcinogenesis of colorectal cancers. To investigate whether O(6)-methylguanine-DNA methyltransferase (MGMT) is altered during colorectal carcinogenesis, we performed immunohistochemical staining on 265 sporadic colorectal cancers, 113 sporadic adenomas, 33 familial adenomatous polyposis (FAP) colorectal cancers, and 93 FAP adenomas. Sixty-seven of 265 sporadic colorectal cancer cases and five of 113 sporadic adenoma cases showed loss of MGMT expression (P < 0.001). Among FAP patients, four of 33 cancers and six of 93 adenomas showed loss of MGMT protein expression. When we compared the association between MGMT promoter hypermethylation and protein expression, almost all cases without a methylated allele were positive for the expression of MGMT. In contrast, cases with promoter methylation frequently showed loss of MGMT expression (P < 0.01). Loss of MGMT was correlated with some clinicopathological characteristics, i.e., tumor invasion (P = 0.013) and stage (P = 0.035) in sporadic colorectal cancer, and degree of atypism (P = 0.042) in sporadic adenoma. Our results show that loss of expression of MGMT occurs more frequently in cancer than in adenoma in both sporadic and FAP patients, and that loss of expression of MGMT is associated with hypermethylation of the promoter area of MGMT gene.     

Inactivation of O(6)-methylguanine-DNA methyltransferase by glucose-conjugated inhibitors.

The DNA-repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) is a decisive determinant of resistance of tumor cells to methylating and chloroethylating anti-cancer drugs. Therefore, selective inhibition of MGMT in tumors is expected to cause tumor sensitization. Several inhibitors of MGMT have been developed which function in both tumors and normal tissue. To deplete MGMT preferentially in tumors, strategies to target the inhibitor to the tumor tissue need to be developed. Here, we report on the properties of glucose-conjugated MGMT inhibitors that might be useful for tumor targeting since tumor cells frequently over-express glucose transporter. O(6)-Benzylguanine (O6BG), 8-aza-O(6)-benzylguanine, O(6)-(4-bromothenyl)-guanine (O6BTG) and the corresponding spacer-linked beta-D-glucose conjugates were analyzed comparatively for MGMT-inhibitory activity. Substitution at the N9 position of the purine moiety resulted generally in a reduction in the efficiency with which the inhibitors blocked MGMT. However, the inhibitory activity of the O6BTG conjugates increased with increasing spacer length, and O6BTG conjugated with a C8 spacer with beta-D-glucose was nearly as effective as O6BTG on its own. MGMT was inhibited by the conjugates both in crude cell extracts and upon treatment of intact HeLa cells, indicating efficient uptake of the glucose conjugates into cells. Since the O6BTG-C8-D-glucose conjugate 8-[O(6)-(4-bromothenyl)-guan-9-yl]-octyl-beta-D-glucoside was highly efficient at MGMT inhibition in a non-toxic concentration range, the drug might be a useful tool for specific tumor sensitization.     

Prognostic impact of O6‐methylguanine‐DNA methyltransferase silencing in patients with recurrent glioblastoma multiforme who undergo surgery and carmustine wafer implantation

BACKGROUND:

O⁶‐methylguanine‐DNA methyltransferase (MGMT) is a key enzyme in the DNA repair process after alkylating agent action. Epigenetic silencing of the MGMT gene by promoter methylation has been associated with longer survival in patients with newly diagnosed glioblastoma multiforme (GBM) who receive alkylating agents. In this study, the authors evaluated the prognostic value of different biomarkers in recurrent GBM and analyzed the changes in MGMT status between primary tumors and recurrent tumors.

METHODS:

Twenty‐two patients who had recurrent GBM and who underwent surgery with carmustine wafer implantation were enrolled prospectively between 2005 and 2007. The authors investigated the correlation between MGMT silencing in the tumor at recurrence and survival taking into account other clinically recognized prognostic factors. MGMT status was determined by using methylation‐specific polymerase chain reaction analysis, a high‐throughput quantitative methylation assay, and immunohistochemistry. In addition, expression analyses of human mutL homolog 1, human mutS homolog 2, and tumor necrosis factor α‐induced protein 3 at recurrence were conducted with regard to their prognostic impact.

RESULTS:

The median progression‐free survival (PFS) and overall survival (OS) rates after recurrence were 3.6 months and 9.9 months, respectively, and the 6‐month PFS rate after recurrence was 27.2%. On multivariate analysis, only age (P = .04) and MGMT promoter hypermethylation at recurrence, as determined by MethyLight technology (P = .0012) and methylation‐specific polymerase chain reaction (MSP) analysis (P = .004), were correlated with better PFS. On multivariate analysis, only MGMT promoter hypermethylation at recurrence, as determined by using MethyLight technology (P = .019) and MSP analysis (P = .046), was associated with better OS.

CONCLUSIONS:

MGMT methylation status was an important prognostic factor in patients with recurrent GBM who underwent surgery plus carmustine wafer implantation; therefore, it was useful in predicting the outcome of GBM therapy at recurrence. Cancer 2009. © 2009 American Cancer Society.     

O(6) -methylguanine-DNA methyltransferase (MGMT) promoter methylation and low MGMT-encoded protein expression as prognostic markers in glioblastoma patients treated with biodegradable carmustine wafer implants after initial surgery followed by radiotherapy with concomitant and adjuvant temozolomide

BACKGROUND:

O⁶‐methylguanine‐DNA methyltransferase (MGMT) promoter methylation status was proposed as a prognostic biomarker for patients with glioblastoma. However, the prognostic impact of MGMT in patients with newly diagnosed glioblastoma who receive carmustine‐releasing wafers (Gliadel) along with temozolomide (TMZ) is still unknown.

METHODS:

MGMT promoter methylation status and protein expression were analyzed in formalin‐fixed, paraffin‐embedded tumor specimens obtained from 111 French patients with newly diagnosed glioblastoma. Patients received the Gliadel wafers followed by radiotherapy plus concomitant and adjuvant TMZ chemotherapy while they were enrolled in a French multicenter prospective study.

RESULTS:

For the whole cohort, the median overall survival (OS) was 17.5 months, and the progression‐free survival was 10.3 months. Patients with tumors that harbored MGMT methylation had a significantly longer OS compared with patients who had wild‐type MGMT (21.7 months vs 15.1 months; P = .025). Similarly, patients who had low MGMT protein expression (≤15%) had a significantly improved OS compared with patients who had high MGMT expression (27.0 months vs 15.1 months; P = .021). The extent of resection was the strongest clinical predictor of outcome. In multivariate Cox models that were adjusted for sex, performance status, and extent of surgery, both MGMT methylation and protein expression were identified as independent prognosticators, and the finding was validated internally using a bootstrap resampling technique. Discrepancies were identified between protein expression and MGMT methylation status, thus suggesting that the 2 assays probably assess different biologic features.

CONCLUSIONS:

MGMT promoter methylation status and low MGMT expression both were identified as positive prognosticators in patients with newly diagnosed glioblastoma who underwent surgical resection and received Gliadel wafer implants followed by adjuvant radiotherapy and concomitant oral TMZ chemotherapy (the Stupp protocol). Cancer 2012. © 2012 American Cancer Society.     

O6‐methylguanine DNA methyltransferase immunoexpression in nonfunctioning pituitary adenomas

BACKGROUND:

Currently, no effective alternative treatment exists for progressive, regrowing, nonfunctioning pituitary adenomas (NFPA) that are resistant to conventional multimodality therapy. Temozolomide (TMZ) was proposed as a treatment option for pituitary carcinomas and aggressive pituitary adenomas. Recently, it was suggested that the responsiveness of pituitary tumors to TMZ depends on the immunoexpression of O⁶‐methylguanine DNA methyltransferase (MGMT). Therefore, the authors of this report assessed MGMT expression in a series of patients with progressive, regrowing NFPAs to evaluate whether TMZ may serve as alternative treatment option.

METHODS:

On the basis of postoperative magnetic resonance imaging, 45 patients with NFPAs were allocated to either a group with progressive, regrowing tumors (n = 24) or a tumor‐free group (n = 21), which served as a control. MGMT expression was assessed semiquantitatively by immunohistochemistry (low expression was defined as ≤50% immunostained adenoma cells, and high expression was defined as >50% immunostained adenoma cells) and was compared between the 2 groups.

RESULTS:

At the time of initial surgery, low MGMT expression was observed in 12 of 24 patients (50%) in the study group with progressive, regrowing NFPAs. In the control group of tumor‐free patients, only 5 of 21 patients (24%) exhibited low MGMT expression. A comparable distribution of MGMT expression was observed in the specimens from repeat surgeries. A shorter interval to second surgery was observed in patients who had low MGMT expression.

CONCLUSIONS:

The current data has suggested that half of the patients with progressive, regrowing NFPAs exhibit low MGMT expression and are potential candidates for treatment with TMZ. These findings provide a rationale for the use of TMZ as an alternative treatment approach in this subgroup if conventional therapy, including reoperation, radiosurgery, and radiotherapy, fails. Cancer 2009. © 2009 American Cancer Society.     

Combined loss of expression of O6-methylguanine-DNA methyltransferase and hMLH1 accelerates progression of hepatocellular carcinoma

BACKGROUND AND OBJECTIVES: O(6)-methylguanine-DNA methyltransferase (MGMT) and human Mut L homologue 1 (hMLH1) are proteins that play an important role in DNA repair. No reports have yet described whether deficient MGMT and hMLH1 expression correlates with tumor progression and the prognosis of patients with hepatocellular carcinoma (HCC). METHODS: Using immunohistochemical analysis, we evaluated the expression status of MGMT and hMLH1 protein in 60 paraffin-embedded samples from consecutive patients with curatively resected HCC. RESULTS: The lack of expression of both MGMT and hMLH1 in HCCs (n = 7) correlated with advanced pTNM stage (P = 0.039), as compared with HCCs expressing both proteins (n = 25). The absence of both MGMT and hMLH1 was a significant indicator of malignant potential. The expression status of both MGMT and hMLH1 was a predictive factor for overall survival in patients with HCC (P < 0.001). CONCLUSIONS: HCC lacking both MGMT and hMLH1 is correlated with an advanced stage and a poor prognosis. The expression status of both repair proteins is a predictive prognostic marker in patients with HCC after surgical resection.     

Contribution of O6-methylguanine-DNA methyltransferase to monofunctional alkylating-agent resistance in human brain tumor—derived cell lines

The DNA repair protein O⁶-methylguanine-DNA methyltransferase (MGMT) has been implicated in resistance of human brain tumors to alkylating agents. We observed that 14 human medulloblastoma- and gliomaderived cell lines differ in sensitivity to the methylating agent N-methyl-N′-nitro-N-nitrosoguanidine (MNNG), as shown by their 28-fold range in 10% survival dose (LD₁₀). By using the substrate analogue inhibitor O⁶-benzylguanine (O⁶-BG), we showed that the contribution of MGMT to resistance varies widely, as evidenced by 3- to 30-fold reductions in LD₁₀ among the lines, and varies up to 20-fold among subpopulations of individual lines. Importantly, variability in resistance, manifested as a 20-fold range in LD₁₀, persists after measurable MGMT is eliminated, disclosing differential contributions of other resistance mechanisms to survival. Cells exposed to MNNG while suspended in growth medium are more resistant than cells alkylated as subconfluent monolayers, and MGMT accounts for a smaller proportion of their resistance. Notably, the MGMT content of the lines is not statistically correlated with MNNG resistance or with potentiation of killing by O⁶-BG, even though MGMT is a biochemically demonstrated determinant of resistance. In contrast, the same lines vary less in resistance to the ethylating agent N-ethylnitrosourea (ENU), and MGMT makes only a small contribution to resistance. Our results strongly indicate that resistance to both MNNG and ENU is multifactorial. © 1995 Wiley-Liss, Inc.     

Expression of human O6-methylguanine-DNA methyltransferase in Chinese hamster ovary cells and restoration of cellular resistance to certain N-nitroso compounds.

We have constructed a plasmid in which the expression of human O6-methylguanine-DNA methyltransferase (MGMT) cDNA is driven by the Rous sarcoma virus promoter sequence. Transfection of this plasmid into Chinese hamster ovary (CHO) cells results in expression of MGMT and in cellular resistance to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and 1-(2-chloroethyl)-1-nitrosourea (CNU), but not to N-nitroso-N-ethylurea. The specific activity of MGMT in transfected CHO cells correlated well with their resistance to MNNG and CNU. Southern analysis showed that the plasmid had been integrated into the CHO cell genome. Western analysis of extracts from transfected CHO cells using an antibody against a peptide corresponding to the carboxyl-terminal end of the human MGMT protein demonstrated a single band with a molecular size of 24-25 kDa; no such band was observed in extracts from wild-type CHO cells. These transfected cells may therefore be used to study the role of MGMT in the repair of alkylating DNA lesions and to determine its importance in carcinogenesis as well as in chemotherapy.     

Predictive impact of MGMT promoter methylation in glioblastoma of the elderly

O(6)-methylguanine-DNA-methyltransferase (MGMT) promoter methylation identifies a subpopulation of glioblastoma patients with more favorable prognosis and predicts a benefit from alkylating agent chemotherapy (CT). Little is known about its prevalence and clinical significance in older glioblastoma patients. We studied 233 glioblastoma patients aged 70 years or more (144 males, 89 females, median age: 74 years, range: 70.0-86.6 years), who were prospectively enrolled in the German Glioma Network, for MGMT promoter methylation by methylation-specific PCR (MSP) in all patients and DNA pyrosequencing in 166 patients. MGMT data were correlated with patient outcome. Median progression-free survival (PFS) was 4.8 months (95% CI: 4.3-5.3) and median overall survival (OS) was 7.7 months (95% CI: 6.3-9.0). MGMT promoter methylation was detected by MSP in 134 patients (57.5%). For the whole cohort, PFS was 5.2 versus 4.7 months (p = 0.207) and OS was 8.4 versus 6.4 months (p = 0.031) in patients with versus without MGMT promoter methylation. Patients with MGMT methylated tumors had longer PFS when treated with radiotherapy (RT) plus CT or CT alone compared to patients treated with RT alone. Patients with MGMT unmethylated tumors appeared to derive no survival benefit from CT, regardless of whether given at diagnosis together with RT or as a salvage treatment. Patients treated with RT plus CT or CT alone demonstrated longer OS when pyrosequencing revealed >25% MGMT methylated alleles. Taken together, MGMT promoter methylation may be a useful biomarker to stratify elderly glioblastoma patients for treatment with versus without alkylating agent CT.     

DNA repair gene O6-methylguanine-DNA methyltransferase: promoter hypermethylation associated with decreased expression and G:C to A:T mutations of p53 in brain tumors

The DNA repair enzyme O(6)-methylguanine-DNA methyltransferase (MGMT) removes alkylating adducts from the O(6) position of guanine and protects cells from cytotoxic and mutagenic effects. Expression of MGMT is decreased in some cancers, which may be the result of methylation of CpG islands of both the promoter and coding regions of the gene. We studied the methylation status of the MGMT promoter in a very large collection of brain tumors (85) using methylation-specific polymerase chain reaction (PCR). Aberrant methylation occurred in 48% of 85 human brain tumor samples. Quantitative real-time PCR showed that expression of MGMT mRNA levels was significantly decreased (P < 0.001) in those brain tumors that had methylation of the promoter region of their MGMT gene. MGMT can prevent G to A mutations by removing alkyl groups from the O(6) position of guanine. We found a significantly increased frequency of G:C to A:T mutations of the p53 gene in brain tumors having a methylated MGMT promoter compared with those having an unmethylated MGMT promoter (P < 0.05), and all the non-CpG dinucleotide G:C to A:T mutations of p53 were in samples with a methylated MGMT promoter.     

O6-methylguanine DNA methyltransferase is upregulated in malignant transformation of gastric epithelial cells via its gene promoter DNA hypomethylation

BACKGROUNDO⁶-methylguanine-DNA methyltransferase (MGMT) is a suicide enzyme that repairs the mispairing base O⁶-methyl-guanine induced by environmental and experimental carcinogens. It can transfer the alkyl group to a cysteine residue in its active site and became inactive. The chemical carcinogen N-nitroso compounds (NOCs) can directly bind to the DNA and induce the O⁶-methylguanine adducts, which is an important cause of gene mutation and tumorigenesis. However, the underlying regulatory mechanism of MGMT involved in NOCs-induced tumorigenesis, especially in the initiation phase, remains largely unclear.AIMTo investigate the molecular regulatory mechanism of MGMT in NOCs-induced gastric cell malignant transformation and tumorigenesis.METHODSWe established a gastric epithelial cell malignant transformation model induced by N-methyl-N’-nitro-N-nitrosoguanidine (MNNG) or N-methyl-N-nitroso-urea (MNU) treatment. Cell proliferation, colony formation, soft agar, cell migration, and xenograft assays were used to verify the malignant phenotype. By using quantitative real-time polymerase chain reaction (qPCR) and Western blot analysis, we detected the MGMT expression in malignant transformed cells. We also confirmed the MGMT expression in early stage gastric tumor tissues by qPCR and immunohistochemistry. MGMT gene promoter DNA methylation level was analyzed by methylation-specific PCR and bisulfite sequencing PCR. The role of MGMT in cell malignant transformation was analyzed by colony formation and soft agar assays. RESULTSWe observed a constant increase in MGMT mRNA and protein expression in gastric epithelial cell malignant transformation induced by MNNG or MNU treatment. Moreover, we found a reduction of MGMT gene promoter methylation level by methylation-specific PCR and bisulfite sequencing PCR in MNNG/MNU-treated cells. Inhibition of the MGMT expression by O⁶-benzylguanine promoted the MNNG/MNU-induced malignant phenotypes. Overexpression of MGMT partially reversed the cell malignant transformation process induced by MNNG/MNU. Clinical gastric tissue analysis showed that MGMT was upregulated in the precancerous lesions and metaplasia tissues, but downregulated in the gastric cancer tissues.CONCLUSIONOur finding indicated that MGMT upregulation is induced via its DNA promoter hypomethylation. The highly expressed MGMT prevents the NOCs-induced cell malignant transformation and tumorigenesis, which suggests a potential novel approach for chemical carcinogenesis intervention by regulating aberrant epigenetic mechanisms.     

Methylation epigenotypes and genetic features in colorectal laterally spreading tumors

Aberrant DNA methylation plays an important role in genesis of colorectal cancer (CRC). Previously, we identified Group 1 and Group 2 methylation markers through genome‐wide DNA methylation analysis, and classified CRC and protruded adenoma into three distinct clusters: high‐, intermediate‐ and low‐methylation epigenotypes. High‐methylation epigenotype strongly correlated with BRAF mutations and these aberrations were involved in the serrated pathway, whereas intermediate‐methylation epigenotype strongly correlated with KRAS mutations. Here, we investigated laterally spreading tumors (LSTs), which are flat, early CRC lesions, through quantitative methylation analysis of six Group 1 and 14 Group 2 methylation markers using pyrosequencing. Gene mutations in BRAF, KRAS and PIK3CA, and immunostaining of TP53 and CTNNB1 as well as other clinicopathological factors were also evaluated. By hierarchical clustering using methylation information, LSTs were classified into two subtypes; intermediate‐methylation epigenotype correlating with KRAS mutations (p = 9 × 10^?4) and a granular morphology (LST‐G) (p = 1 × 10^?7), and low‐methylation epigenotype correlating with CTNNB1 activation (p = 0.002) and a nongranular morphology (LST‐NG) (p = 1 × 10^?7). Group 1 marker methylation and BRAF mutations were barely detected, suggesting that high‐methylation epigenotype was unlikely to be involved in LST development. TP53 mutations correlated significantly with malignant transformation, regardless of epigenotype or morphology type. Together, this may suggest that two molecular pathways, intermediate methylation associated with KRAS mutations and LST‐G morphology, and low methylation associated with CTNNB1 activation and LST‐NG morphology, might be involved in LST development, and that involvement of TP53 mutations could be important in both subtypes in the development from adenoma to cancer.     

MGMT在脑胶质瘤组织中的表达及其与患者生存期的关系

背景与目的:目前的研究已经证实DNA修复酶——6-氧-甲基鸟嘌呤DNA甲基转移酶(O6-methylguanine-DNAmethyltransferase,MGMT)在脑胶质瘤组织中的表达与肿瘤的耐药性有一定的关系,并且能够影响肿瘤的化疗效果。本研究通过分析MGMT在脑胶质瘤组织中的表达及其与患者生存期的关系,为基于耐药机制上的脑胶质瘤分子分类提供参考资料。方法:用组织芯片技术和免疫组织化学方法检测311例脑胶质瘤石蜡标本中MGMT的表达情况,并对所有患者进行手术后的5年随访。结果:MGMT表达阳性者126例,占40.51%(126/311)。其中,星形细胞瘤中阳性率为50.41%(61/121),少枝胶质细胞瘤中为25.71%(18/70),少枝星形细胞瘤中为28.13%(18/64),胶质母细胞瘤中为51.79%(29/56);在Ⅰ～Ⅱ级胶质瘤中,MGMT表达的阳性率为36.56%(68/186),而在Ⅲ～Ⅳ级胶质瘤中为46.40%(58/125),经χ2检验分析,两者之间有显著性差异(P<0.001);将MGMT的表达与患者生存期的关系绘制成Kaplan-Meier生存曲线,并进行log-rank分析,MGMT表达阳性者与阴性者之间的差异有显著性(P<0.05)。结论:MGMT在脑胶质瘤的异常表达与肿瘤的组织类型、病理级别有关,MGMT表达阳性患者的生存期明显低于表达阴性的患者。     

O6-Methyguanine-DNA Methyl Transferase (MGMT) Promoter Methylation in Serum DNA of Iranian Patients with Colorectal Cancer

Introduction: Colorectal cancer (CRC) is a leading cause of cancer deaths worldwide but current molecular targetedtherapy is not providing major success in CRC treatment, so early detection by non-invasive methods continues tobe vital. Aberrant methylation of CpG islands in promoter regions is associated with inactivation of various tumorsuppressor genes. O6-methyguanine-DNA methyltransferase (MGMT) is a DNA repair enzyme that removes mutagenicand cytotoxic adducts from O6-guanine in DNA. Aberrant hypermethylation of the MGMT promoter has beenassociated with lack of mRNA expression, with concomitant loss of protein content and enzyme activity. AIM: Ouraim was to determine whether MGMT promoter methylation might be detectable in circulating free DNA in the serumof CRC patients and normal individuals using a methylation specific (MSP) polymerase chain reaction (PCR) method.Methods: A total of 70 subjects were enrolled in the study. Of these, 30 patients who were diagnosed previously asuntreated colon adenocarcinoma by a gastroenterologist and the other 40 were nearly age-matched individuals who hada normal colonoscopic evaluation (except for hemorrhoids or fissures) and normal pathologic reports. After bisulphitemodification of DNA, serum samples were examined for MGMT promoter methylation using MSP. Results: Ninetypercent of CRC patients had MGMT promoter hypermethylation as compared to no methylation in normal subjects’serum. Most of the cancers were stage П and moderately differentiated adenocarcinomas; nearly 60% were found inthe left colon. No statistically significant correlation was found between the promoter methylation status and genderand age. Discussion and Conclusions: MGMT hypermethylation can be detected in free circulating DNA in serum ofCRC patients and can be used “as a clinical biomarker” for early diagnosis and prognostic assessment of the disease.Our data confirm previous studies indicating utility for free circulating DNA as a serum biomarker for early detection,diagnosis and monitoring of CRC patients.     

Role of O6-methylguanine-DNA methyltransferase and effect of O6-benzylguanine on the anti-tumor activity of cis-diaminedichloroplatinum(II) in oral cancer cell lines

The DNA repair enzyme O⁶-methylguanine–DNA methyltransferase (MGMT) modulates the effectiveness of alkylating agents. However, the relationship between MGMT and the sensitivities to other agents has not been explored. In the present study, the association between MGMT expression and the cellular sensitivity to the platinum agent, CDDP was examined in four human oral cancer cell lines. CDDP depleted MGMT protein and mRNA levels in all four cell lines. Two cell lines with low MGMT expression were sensitive to an alkylating agent, N-methyl-N′-nitro-N-nitrosoguanidine and CDDP, whereas two other cell lines with high MGMT expression were resistant to both agents. Furthermore, the addition of the MGMT inhibitor, O⁶-benzylguanine (O⁶-BG), invariably enhanced CDDP sensitivity. CDDP depleted MGMT expression, and CDDP sensitivity was enhanced by O⁶-BG. These results provide valuable information about the relationship between MGMT expression and CDDP sensitivity in oral cancer chemotherapy.     

KRAS 基因突变与结直肠癌转移模式相关性的研究简

目的：研究结直肠癌患者KRAS基因突变与根治后转移的相关性。方法：检测132例结直肠癌根治术患者石蜡组织中的KRAS基因,回顾性分析患者根治术后3年内转移、复发情况,采用单因素及多因素分析KRAS突变与转移的相关性。结果：各转移组KRAS基因突变率分别为：肺（63．2％）、脑（66．7％）、肝（30．4％）及无转移组（34．7％）。单因素分析提示肺、脑转移组KRAS基因突变率显著高于肝转移组及无转移组（P〈0．05）。纳入年龄和性别校正后采用Logistic回归分析提示KRAS突变显著增加肺转移及脑转移风险（P〈0．05）,而肝转移风险无显著升高（P〉0．05）。结论：结直肠癌转移模式与KRAS突变有关,检测KRAS突变状态有助于更加有效地制定复发监视策略。     

Contribution of O6-methylguanine-DNA methyltransferase to resistance to 1,3-(2-chloroethyl)-1-nitrosourea in human brain tumor-derived cell lines

To assess the possible role of the DNA repair protein O⁶-methylguanine-DNA methyltransferase (MGMT) in resistance of brain neoplasms to the clinically important chloroethylating agent 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), we quantitated MGMT activity, BCNU survival, and the effect of ablating MGMT activity on the sensitivity of 14 human medulloblastoma- and glioma-derived cell lines. BCNU resistance, measured as 10% survival dose (LD10), differed eightfold among the lines. Elimination of measurable MGMT activity with the substrate analogue inhibitor O⁶-benzylguanine (O⁶-BG) revealed a variable but limited contribution of MGMT to survival. In no case did O⁶-BG reduce LD₁₀ by more than 3.4-fold. In contrast, 06-BG reduced the LD10 for N-methyl-N′-nitro-N-nitrosoguanidine up to 31-fold in the same cell lines (Bobola MS, Blank A, Berger MS, Silber JR, Mol Carcinog 13:70–80, 1995). Variability in BCNU survival, manifested as a sevenfold range of LD₁₀, persists after measurable MGMT was eliminated, indicating that another mechanism or mechanisms is operating to limit cytotoxicity. Cells alkylated while suspended in growth medium are more resistant to BCNU and display less dependence on MGMT than cells treated while proliferating on a plastic substratum. When alkylated in suspension, most of the lines are either unresponsive to O⁶-BG or contain a subpopulation that did not respond to O6-BG. Our results demonstrate that BCNU resistance is multifactorial and that MGMT makes a modest contribution to resistance in our lines. © 1995 Wiley-Liss, Inc.